1. Field of the Invention
The ARGOS system is a joint U.S./French space program utilizing orbiting satellite receivers and ground based beacons called platform transmitter terminals (PTTs). The ARGOS system provides data collection and platform location services on a global scale. The present invention is an improved ARGOS platform transmitter (PTT) that can be used with the ARGOS or similar systems and has reduced size, weight and extended life due to a solar array/rechargeable battery power source operating under the supervision of a unique power supply control and protection means. The invented ARGOS (PTT) increases the usefulness of the ARGOS system in tracking migratory birds, animals, buoys and balloons, etc. on a global basis. The invented PTT opens the possibility for tracking wild animals weighing as little as a few kilograms.
2. DESCRIPTION OF THE CONTEMPORARY AND/OR PRIOR ART
The ARGOS system utilizes the concept of Doppler frequency shift to calculate the position of transmitting beacons (platform transmitter terminals (PTTs)) on a global basis. A discussion of the principles of the ARGOS system is found in the following article, which is incorporated herein by reference: Bessin, J. L., "Operational Data Collection and Platform Location by Satellite", Remote Sensing of the Environment, Vol. 11, pp. 93-111, 1981. The ARGOS system is composed of three elements: (1) the user's sensors and transmitters (known as platform transmitter terminals or PTTs); (2) two satellites in orbit at any one time; and (3) a center, located in Toulouse, France, to process data and distribute results. A description of the ARGOS system can be found in the following guide, published by Service ARGOS, which is incorporated herein by reference: ARGOS Users Guide, Service ARGOS--Centre National D'Etudes Spatiales, Toulouse, France. Each PTT transmits at 401 MHz and sends a message, less than one second in duration, every 40 to 200 seconds, depending on the transmitter.
The two satellites currently in the Argos system are the NOAA-6 and NOAA-7, both in near-polar orbits at approximately 850 km altitude, periods of 102 minutes and spaced roughly 90.degree. apart in longitude. With these characteristics one can expect 6-8 passes per day at the equator (each of these passes are long enough to locate the PTT). A maximum of 28 passes is possible in the polar regions since each satellite orbits fourteen times each day.
The basic principle of the Argos system is that the PTT can be located by measuring the Doppler shift of the PTT frequency during the satellite pass, typically 10-15 minutes duration. Distinguishing between various PTTs is done by encoding an identification number in the signal. Handling signals from many PTTs is possible since each PTT is "on" less than one second each minute and the arrival time of signals at the ARGOS receiver is random. In addition, up to four signals can be handled simultaneously provided they are shifted to different frequencies. Literally hundreds of PTTs can be handled worldwide.
Processing the data is performed by Service Argos in France, which computes the PTT location and will convert sensor data into engineering units. Current user fees are $20 per day per platform for location determination services. Each platform (PTT) is assigned a unique identification code by Service ARGOS. To date, nearly all PTTs have been located on balloons, buoys, icebergs, or ships.
To be utilized in migratory bird and animal tracking, and to be better utilized in buoys and balloon applications, the PTT must have: (1) a small volume and low weight, (2) sufficient power to be detected by a satellite and, (3) a long lifetime. These are conflicting goals which are difficult to satisfy, and have not been satisfied in the prior art. A large portion of the weight of any transmitter is the power supply. There are two choices for the power supply: use primary (non-rechargeable) batteries or secondary (rechargeable) batteries plus solar cells. Using primary cells that have sufficient energy to power an ARGOS PTT for a 200-day life time will seriously increase the transmitter's weight budget. The use of solar cells combined with rechargeable batteries reduces the transmitter's weight but produces an erratic and unstable power source because of vagaries of the weather and habits of birds or animals, if the PTT is used in migratory tracking. Such an unstable power supply would cause the PTT to frequently turn "off" and "on" with changes in available energy from the solar array/rechargeable battery power supply. This is undesirable since the PTT must transmit continuously for at least 2 hours to assure "visibility" by an ARGOS satellite. Since these problems had not been solved, prior art ARGOS PTT systems weigh approximately 2 Kilograms and are too heavy for many applications.